The W123 and W126 '85 models (300d and 300SD respectively) are all
iron.  Infact, the engines are the same on both chasis.  If you want an
"indestructable" engine, you want OM617 turbo.  I am only familiar with
1985 300D and 1985 300SD as I have owned/own both of them.  W123/W126
OM617 Turbo was produced from 81-85 if memory serves me right.  '86 and
later models had aluminum heads.  1985 was the last year of the 617
with some refinements to almost perfect the design.

Based on my ownership experience of 1985 300D and 300SD, they both are
equally reliable.  Besides being an entirely different chasis, the W126
has some more "electronics" (power seats, electric components for
locks, etc).  Other than that they seem pretty much the same.  However,
I prefer the W126 ride much better.  W126 is a little more roomy
comapred to W123.

Don't let anyone fool you into thinking that diesels last forever.
Nothing lasts forever.  However, with proper maintanence, these engines
have known to easily go 500K miles.  Compression is not the only thing
to look at.  I would look for all maint records.  Diesels require that
the engine oil was changed religiously.  The oil is what keeps the soot
in suspention.  If oil changes are ignored, it is well know that the
engine will wear much quicker.

check out http://www.mercedessource.com/  and look at "maint alert"
tab.

good luck.

Anil Bharucha
1985 300SD 217K mi

In reverse order, one connects a compression tester to the diesel's glow
plug bores which are M 12 x 1.25. The specification for a 617.95X (5
cylinder all iron engine) is 319 - 348 psi, 218 psi minimum and 44 psi
allowable difference between cylinders. The engine's valves ought to be
in adjustment or the compression test is meaningless. The 617.95X
turbodiesel was manufactured for MY '78 through '85 and is a very tough
durable machine. But has mechanical valve adjustment that ought to be
done every 15 K miles. These motors made 120 HP and about 170 ft lbs
torque at 2,400 rpm.

The next newer engines ('86 & '87 MY) were straight sixes with hydraulic
valves; they produced 148 HP and are smoother, more modern designs but
not as tough due to their aluminum heads. These engines cannot be
overheated. These sixes were installed in the 300D (124) and 300SDL.

The cars you are considering are old and most have done a zillion miles
by now. Of course you and others will find exceptions but most cars are
quite worn after 20 years use. So rather than focusing on certain types
123 vs 126 etc. I suggest you look for a car that's in good condition. I
would personally avoid old cars with twenty to twenty four year old air
bags. These old airbags - if they even still deploy - were contemporary
in 1980s but are crude by today's dual stage standards. Remember,
airbags were mandated because seat belts were not being used, not
because seat belts failed to restrain their drivers and passengers.

Finally, don't chase after diesel cars just because they're diesels.
Sellers know this too and raise the asking price accordingly. Look at a
model's gasoline version's asking prices, figure the annual fuel use and
added gasoline cost and whether it even makes sense to buy an old diesel
just because it's diesel powered. A minor collision often "totals" an
old car because insurance companies will only pay book value and a car's
market at that age is not very high (plus, insurers' "independent
surveys" find the lousiest old dogs, cite them as "market sales" and
those sales are 2 to 6 months old so one can no longer inspect those
cars' condition to challenge the price - they're long gone - and that
ends the negotiation).

Biodiesel seems to be driving the choice of car. Should it?

300SDL is 4-5" stretched than 300SD.  All the extra space is allocated
to rear passenger so you can imagine it is like a limo ride back there.
 300SDL was made for 86 and 87 only and that was when aluminum head
started.  If you are looking for iron head (which outlasts everything
else on the car :-), stay with 300SD (or W123) and no later than 85.

If you want a diesel wagon, that is W123 only.  W124 has wagon too but
that is aluminum head.

In fact, aluminum head is OK as long as it is not overheated.  Of
course, iron head can withstand mistake which means bullet proof
reliability.

The 6 cylinder engines were the ones with aluminum heads, the 5's had
iron heads.

The 300SDL had an aluminum head, the 300SD didn't.
The 350SDL did.

The 190D 2.5 did.

No.

The 126's are less reliable as they have more stuff. Electric everything;
more parts means more things can fail.

4" more rear legroom. Same as the difference between a 500SE and a 500SEL.

Diesel compression tests are not a DIY thing. Diesel fuel at that pressure
can cause gangrene. You take it to a diesel shop.